The role of feto-placental unit in the maintenance of pregnancy is studied by (1) examination of placental gonadotrophins for their secretory patterns, steroidogenic activity and immunoreactivity in the subhuman primates, and laboratory animals, and (2) investigation of the role of fetal endocrine organs in the onset of parturition. Placental hormones which stimulate the luteal function are isolated and characterized for their steroidogenic activity. Antisera to these hormones are produced and the immunoreactivity of the antibodies is characterized. The effect of these antisera on ovarin steroid hormone secretion is studied. The role of fetal endocrine organs in the onset of parturition is studied by surgical intervention and/or hormone therapy to pregnant subhuman primates and laboratory animals near term.